How many contracts?

I know this sounds like a total newb question, but how can I calculate the maximum number of e-mini S&P contracts I can hold intraday on collective2 given my current buying power? I thought I had run the numbers correctly but I had my order rejected today for lack of buying power so apparently my margin requirements are wrong.

For a concrete example, what is the maximum number of @ESZ8 contracts I could hold intraday with a buying power of $85,410?